The overall goal of this work is to study the autonomic control of atrioventricular (AV) and ventriculoatrial (VA) conduction. Two projects are concerned with potential vagal contributions to common arrhythmias, and the third project is a detailed study of vagal-sympathetic-heart period interactions on AV conduction. Specifically, the AV nodal Wenckebach arrhythmia will be studied as a function of: a) brief vagal stimulus bursts and the timing of these bursts in the cardiac cycle, b) the time after the onset of vagal stimulus trains, c) background sympathetic activity, d) the interaction of basic cycle length (heart period) and a) above, and e) a comparison of VA vs. AV conduction for a) to d) above. Secondly, facilitated conduction of a premature depolarization will be studied. That is, with AV sequential pacing the AV junctional refractory period is "peeled back," and a premature depolarization (PD) is conducted more rapidly (facilitated) than with single chamber pacing. Facilitated conduction will be studied as a function of: a) basic cycle length of the paced beats and coupling intervals of the premature beats, b) brief bursts of vagal activity and the timing of that vagal activity in a cardiac cycle, c) different levels of sympathetic activity, and d) the interaction of a) and b) above for both AV and VA conduction. The third project concerns the effects of brief bursts of sympathetic activity, and the dynamic beat-by-beat interactions of vagal activity, sympathetic activity, and heart period on AV and VA conduction. In each dog, each effect will be studied separately and in various simultaneous combinations to provide quantitative conclusions. The results of these three projects should provide basic new insights into the physiology of the control of AV conduction, and how the control of VA conduction importantly differs. This information is additionally important for the design and clinical optimization of artifical cardiac pacemakers where serious problems now arise from our poor understanding of these phenomena.